Mdi Production By Means of Liquid Phase and Gas Phase Phosgenation

Abstract

The invention relates to a process for preparing isocyanates, which comprises the ( 1 ) preparation of a crude MDA mixture by reaction of aniline with formaldehyde, ( 2 ) separation of the crude MDA mixture into MMDA and PMDA, ( 3 a) phosgenation of the PMDA separated off in step 2 in the liquid phase to form PMDI and ( 3 b) phosgenation of the MMDA separated off in step 2 in the gas phase to form MMDI.

1 - 4 . (canceled) 
     
     
         5 . A process for preparing isocyanates, wherein said process comprises:
 (1) preparing a crude MDA mixture by reacting aniline with formaldehyde;   (2) separating said crude MDA mixture into a gas phase comprising MMDA and a liquid phase comprising PMDA;   (3a) phosgenating said PMDA in said liquid phase separated off in (2) to form PMDI; and   (3b) phosgenating said MMDA in said gas phase separated off in (2) to form MMDI.   
     
     
         6 . The process for preparing isocyanates according to  claim 5 , wherein said reacting of aniline with formaldehyde in (1) is carried out at a ratio of aniline to formaldehyde ranging from 2 to 5.5. 
     
     
         7 . The process for preparing isocyanates according to  claim 5 , wherein said separating of said crude MDA mixture in (2) is carried out so that said MMDA separated off comprises a maximum content of PMDA of 12 wt. %. 
     
     
         8 . The process for preparing isocyanates according to  claim 5 , wherein said separating of said crude MDA mixture in (2) is carried out so that said PMDA separated off comprises a maximum content of MMDA of 30 wt. %.